SharedPreferences字符串设置应用程序终止后丢失的数据（Android模拟器）

Question

I am running this on an emulator: 5554:Nexus_5_API_22_x86.

I am trying to learn SharedPreferences and have written a simple test program.

It contains two buttons: one adds a String + random # to a set which will be stored in SharedPreferences, and the other prints the contents of that set.

Whenever I press the square button on the bottom right hand of the screen and press 'x' to close the app window, then relaunch the app, the contents of the set are reset - in other words, printing the set yields nothing.

However, if I exit the app using only the back button, the contents remain - in other words, printing the set yields whatever was in it before.

Java:

...

public class MainActivity extends AppCompatActivity
{
    final int PREF_MODE_PRIVATE = 0;
    TextView output;
    Set<String> testSet;
    Random randomGenerator = new Random();
    SharedPreferences data;
    SharedPreferences.Editor dataEditor;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        output = (TextView) findViewById(R.id.textView); //getting the output textfield
        data = getPreferences(PREF_MODE_PRIVATE);

        //If first-time setup has not been completed, execute the following block
        //I don't want the String Set to be reset to empty every time the app is launched
        if(data.getBoolean("initialized", false) == false)
        {
            //Adding the empty set to storage
            testSet = new HashSet<String>();

            dataEditor = data.edit();
            dataEditor.putStringSet("testSet", testSet); //Add the empty Set to storage
            dataEditor.putBoolean("initialized", true); //Set initialized flag to true
            dataEditor.apply();
        }
    }

    public void printTestSet(View view)
    {
        output.setText(""); //Clears the text field
        Set<String> toBePrinted = data.getStringSet("testSet", null); //Gets the String Set

        //Prints content of the String Set
        if(toBePrinted != null)
        {
            for(String word : toBePrinted)
            {
                output.append(word + '\n');
            }
        }
    }

    public void addToTestSet(View view)
    {
        //Generate a string followed by a random number and add it to the String Set
        int randomInt = randomGenerator.nextInt(1000);
        data.getStringSet("testSet", null).add("NEW STRING #" + randomInt);
    }
}

The button that prints the String Set calls printTestSet and the one that adds a String to the Set calls addToTestSet .

Upon creation, the app uses a simple boolean to check if it has been initialized the for the first time. If not, it adds an empty String Set to storage and sets the boolean to true. If the boolean is already true (meaning it has already added the empty string set), that step is skipped.

Answer 1

It looks like you're not saving the shared preferences in addToTestSet. When you do getStringSet and then add, you need to again save the string Set back into shared prefs like you do in your onCreate() using dataEditor.apply() .

Or if you want to be a bit more efficient, you can save your stringSet in the activity's onPause() method to prevent constantly writing to SharedPrefs

When you hit back, your app process isn't being killed, which means when you open it up again the Android system is restoring whatever it can (what was written in textViews , checkboxes that were checked, etc. simple things like that). What you see in the box might not actually be getting populated by SharedPrefs .

Answer 2

You need to either commit your data either realtime (where you are doing apply) or in application life cycle handler onpause (when your app goes to background). Use option 1 when you have little data or 2 when you have large amount of data to commit.